Ion source having both deflection and repeller electrodes for directing an electron stream



J. L. PETERS 3,296,481 ION SOURCE HAVING BOTH DEFLECTION AND REPELLERELECTRODES Jan. 3, 1967 FORDIRECTING AN ELECTRON STREAM Filed May 15,1964 Rm m .w w Ne w w A N L 7w I n h I 2 a 3 3 3 2)7.\ 313 3 I THW w m 26 3 V O 0 0 V 7 53 1211?. o w w 4 4 2 4. r 2 m m m m m m n m k m3 1 n nI m m Q G W "w. m 3 3 III I. 21L 4 4 w t O m D m 2 Wm /l\ c D. a w GAGENT United States Patent 3,296,4551 ION SGURCE HAVENG BQTH DEFLETIONAND REPELLER ELECTRODES F012 DKRECTING AN ELEQTRON STREAM John L.Eeters, 114 Dikeman St Hempstead, NY. 11550 Filed May 13, 1%4, Ser. No.367,644 11 (Ilaims. (Cl. 313230) The present invention relates to ionsources and, more particularly, to means for producing a positive ionbeam.

In apparatus such as analytical mass spectrometers and mass spectrometerleak detectors, for example, a source of positive ions and a means forfocusing and accelerating the ions are employed to produce a positiveion beam. One type of widely used ion source comprises a box-like ionchamber that contains molecules of gas, means for producing anddirecting a beam of electrons into the chamber, an ion outlet at thebottom of the chamber, and a grid positioned at the top of the chamberfor repelling ions toward the ion outlet. The production of usable ionsand, consequently, the current in an ion beam that is formed from theions leaving the ion outlet depends upon the number of electrons andfocusing or controlling of the electrons within the ion chamber.

It is a primary object of the present invention to provide improvedmeans for producing a positive ion beam with increased current.

Another object is to provide an improved ion source for increasing theusable positive ions that are produced in the ion chamber of the ionsource.

Still another object is to provide improved means for focusing orcontrolling an electron beam in an ion chamber of an ion source forincreasing the usable positive ions which are produced in the chamber.

A further object is to provide improved means for preventingcontamination in an ion source so that a maximum amount of usable ionscan be produced.

The foregoing objects as well as other objects and advantages of theinventtion are achieved by an ion source, as is described above, towhich is added a deflection electrode that is positioned adjacent theion repelling grid on the far side of the grid from the bottom of theion cham ber. The focusing electrode and the grid are operated atdifferent voltages to optimize the trajectories of the elec trons in theionization chamber thereby increasing the number of positive ions thatare produced in the chamber. An internal heater is positioned in back ofthe focusing electrode. Means are provided for turning the heater offwhen the filament of the means for producing and directing a beam ofelectrons into the chamber is switched on and vice versa, therebypreventing contamination.

In the drawings,

PEG. 1 is a sectional view, partly schematic of ion beam producing meansthat is provided in accordance with the present invention;

FIG. 2 is an end view, taken from the line 22 in FIG. 1, of the focusingelectrode that surrounds the filament of the electron stream producingmeans;

FIG. 3 is a sectional view, which is taken at right angles with the viewshown in FIG. 1, of the ion beam producing means; and

FIG. 4 is an enlarged plan view, taken from the line 44 in FIG. 3, ofthe repeller electrode.

Referring to FIGS. 1 and 3, there is shown a positive ion beam producingmeans comprising an ion chamber 11 that contains molecules of gas, meansincluding a filamerit 12 and focusing electrode 13, shown in FIG. 1, forproducing and directing a beam of electrons into the chamber 11 forproducing positive ions, at repeller electrode 16 positioned at the topof the chamber for repelling positive ions toward the bottom of thechamber, and the combination of focusing electrodes 18a and 18b and anaccelerating electrode 19 positioned below the ionization chamber forfocusing and accelerating an ion beam that is formed from the ions whichpass out of the ion chamber. In accordance with important features ofthe inention, a deflection electrode 22 is positioned in back of therepeller electrode 16 on the far side of the electrode 1e from thebottom of the ionization chamber and a heater 23 is positioned in backof the deflection electrode 22.

The heater 23 is a coil or zig zag heater winding which is insulatedfrom the electrode 22. A heat shield 26, made from polished metal, ispositioned in back of the heater 23 for reflecting heat back into theion source. The heater 23 is energized by a voltage supply 25 which maybe either a DC. supply as is indicated in FIG. 1 or an AC. supply. Theelectrode 22, which is a plate made from non-magnetic metal like all ofthe parts of the ion source, distributes the heat produced by heater 23.

Suitable insulator means, not shown, are employed to support the variouselements of the ion beam producing means in insulated relationship witheach other within a vacuum envelope 20 that is made from eithernonconducting material or from metal. A vacuum pump, which is identifiedin FIG. 3, is connected to the envelope 2% for drawing gas from the gasinlet, which is identified in FIG. 3, through the ion chamber 11.

The ionization chamber 11 is a rectangularly shaped box-like electrode.It is open at the top and is closed at the bottom except for arectangularly shaped slit 24 which forms an ion outlet from the chamber.One of the narrow side walls 27 of the chamber 11 contains arectangularly shaped slit 28 for passing a beam of electrons into theionization chamber.

The beam of electrons which passes into the ionization chamber 11 isproduced by the filament 12 and the focusing electrode 13. The filament12 is a rectangularly shaped strip of tungsten, for example, which isapproximately twice as wide as filaments heretofore employed in ionsources. The increased surface area of the wider filament directs moreelectrons into the ionization chamer 11 to produce more usable positiveions to produce a larger current in the positive ion beam, compared withfilaments which have been used in the art heretofore. The filament 12 isenergized by a voltage supply 29 shown in FIG. 1. A DC. voltage supplyis shown, but instead an A.C. supply could be used.

The focusing electrode 13 is shaped to form an electron stream that hasa rectangular cross section A suitable design for this focusingelectrode is shown and de scribed in an article entitled MassSpectrometer Leak Detector With Improved Sensitivit by John L. Peters,on page 1094 of the Review of Scientific Instruments, vol. 30. N0. 12,December 1959, for example. The focusing electrode 13 is supportedmechanically by struts 34 and 35 which extend from a fiat plate 36 thatis supported within the tube envelope 20 by suitable insulator means,not shown.

As is shown both in FIG. 1 and in FIG. 3, the various electrodes of theion source are connected respectively to a plurality of adjustable taps(represented by the arrow heads) along a resistor 30. The resistor 30 isconnected across the output terminals of a regulated DC. voltage supply,not shown. One end of the resistor is connected to ground, for example,the other end of the resistor being at a positive voltage of 700 volts(for example) with respect to ground.

The repeller electrode or grid 16 comprises a metal plate or frame 31and fine wires 32 which are stretched across a rectangularly shapedaperture 33. The wire portions are accurately positioned and firmlyanchored to the frame 31 by welding, for example. The repeller electrode16 is biased by connecting it to the adjustable tap 37 so that electrode16 is approximately ten volts more positive than the voltage of the ionchamber 11. The chamber 11 is connected to the resistor 30 by anadjustable tap 38. As an example, the repeller electrode 16 containsapproximately ten wires 32 from .005 inch to .010 inch in diameter, madefrom Nichrome V, which are spot welded at each end to the frame 31 andare spaced evenly apart by approximately .060 inch. In constructing theeiectrode 16 the wires 32 are straightened, stress relieved and thenheld straight under slight tension when welded.

Beyond the repeller electrode 16 on the far side of the repeller fromthe bottom of the ion chamber 11, the second focusing electrode 22 issupported in insulated relationship with respect to the repellerelectrode. The deflection electrode 22 is a flat metal plate that isinsulated both from electrode 16 and from the first focusing electrode13 so that operating voltages of these electrodes can be adjustedseparately for obtaining optimum trajectories of the electrons in theion chamber 11 for producing a maximum number of positive ions. Thedeflection electrode 22, the heat shield 26 and the filament 12 all areconnected to adjustable tap 40 which is approximately -50 to 100 voltsmore positive than the potential of the focus electrode 13 at tap 41.The distance from the second deflection electrode 22 to the repellerelectrode 16 is chosen so that the electrode 22 can be operated atfilament potential. This eliminates the need for another control andanother adjustment. This distance is determined experimentally and is0.100 inch, for example, in an operative embodiment of this invention.

The heater element 23 behind the focusing electrode 22 indirectly heatsthe electrode 22 and the whole ion source. About watts of power, forexample, is supplied to the element 23 by the power supply 25 throughthe switch 38. About 20 watts of power also supplied to the filament 12by the power supply 29 through the switch 39.

In accordance with one of the features of this invention, the switches29 and 38 are ganged, as is illustrated by the dashed line 44 in FIG. 1,so that the heater 23 is turned off when the filament 12 is switched on,and vice versa. This provides approximately constant ion sourcetemperature and sufficient heat at all times to prevent contamination ofthe ion source regardless of whether or not the filament 12 is on or is01f. External heater elements, not shown, also are mounted on the tubeenvelope at positions where contamination otherwise would occur. Thetemperature of the housing in the ion source region is maintained atfrom 80 degrees to 90 degrees centigrade, for example. By heating theion source, as is described above, contamination is prevented and thespectrometer tube, in which the ion source is used, for example, willretain its sensitivity and optimum performance for a considerable lengthof time.

By using the two heaters (filament 12 and coil 23) alternately, amaximum usable amount of heat is put into the ion source at all times.In prior art ion sources the heat for decontaminating the ion source isproduced both by a hot repeller and by the filament of the electron beamproducing means. Therefore, the watts of power supplied to the repelleradded to the watts of power supplied to the filament must be the maximumallowable. In this prior art ion source when the filament is off (whichis most of the time, nights, etc.), maximum heat to avoid contaminationis not being used.

In operation, when the filament 12 is energized by closing the switch 39there is produced a beam of electrons 42 having a rectangular crosssection. The beam is shaped and accelerated by the focusing electrode 13and by the accelerating electrode formed by the boundary of slit 28. Asthe electron beam 42 crosses the ion chamber 11, it collides withmolecules of gas which come into 4. the chamber from the gas inlet, FIG.3, to produce positive ions.

Within the chamber 11 there is a tendancy for the electrons to penetratethe repeller electrode 32 to be partly collected and partly deflectedback into the ion chamber 11. By providing a focusing electrode 22behind the repeller electrode 16 and by adjusting the tap 40 to whichelectrode 22 is connected, the trajectories of the electron streamwithin the ion chamber 11 are controlled for achieving an optimum rateof ion production and an ion beam having maximum current.

The positive ions produced in the ion chamber 11 are repelled toward thebottom of the chamber by the repeller electrode 16. The positive ionsare formed into a beam having a rectangular cross section which passesthrough the rectangularly shaped slit 24 at the bottom of the ionchamber through the focusing electrodes 18a and 18b, and through theaccelerating electrode 19. The ion beam is wedge-shaped and comes to aline focus in the vicinity of the accelerating electrode 19. Below theaccelerating electrode 19 the ion beam passes through a rectangularlyshaped slit 46 in an iron bafile 43 which is provided for interceptingstray ions. The baffle 43 is made from metal and is connected to groundas is illustrated in FIG. 3.

The improved ion source, as is described above, produces an order ofmagnitude increase in positive ion current with a corresponding increasein sensitivity of a mass spectrometer using this improved source.Optimum performance is maintained by avoiding contamination byalternately heating the source by the internal heater 23 and by thefilament 12 as is described above.

Since changes could be made both in the illustrated embodiments of theinvention and in the above description, and different words ofdescription might be used without departing from the scope and spirit ofthe invention, it is understood that the invention is limited solely bythe accompanyng claims.

What is claimed is:

1. An ion source comprising an ion chamber, means including a filamentand first focusing electrode for producing and directing a stream ofelectrons into said chamber, an ion outlet on one side of said chamber,a grid positioned on the opposite side of said chamber for repellingions toward said one side, and a deflection electrode positioned on thefar side of said grid from said one side of said chamber andsubstantially parallel to said grid for controlling the trajectories ofelectrons in said chamber to increase the amount of positive ions whichare produced in said chamber, said deflection electrode imposingoperative field forces in said chamber through the interposed grid.

2. The ion source as is set forth in claim 1, in which said deflectionelectrode is physically separated from said grid.

3. The ion source as is set forth in claim 2, further including meansfor supplying dilferent voltages to said deflection electrode and saidgrid.

4. The ion source as is set forth in claim 1, further including a heaterpositioned behind said deflection electrode, and means for energizingsaid heater and said filament alternately for maintaining anapproximately uniform temperature in the ion source for preventingcontamination.

5. An ion source comprising an ionizing chamber, means including afilament and an electron focus elec trode positioned laterally of saidchamber for producing and directing an electron stream into said chamberto produce positive ions, an opening in the bottom of said chamber forpassing ions out of said chamber, a repeller electrode positioned at thetop of said chamber, said repeller electrode comprising a plurality ofspaced conductive elements between which electrons can escape from saidchamber, and a deflection electrode positioned above said repellerelectrode, spaced physically therefrom in substantially parallelrelation, and operative therethrough for controlling the trajectories ofthe electron stream in said chamber.

6. An ion source as is set forth in claim 5, further including a heaterpositioned behind said deflection electrode, and means for energizingsaid heater and said filament alternately for maintaining anapproximately uniform temperature in the ion source for preventingcontamination.

7. An ion source comprising an ion chamber having a bottom wall andupwardly extending side walls, an aperture in one of said side walls,means for producing and directing a stream of electrons through saidaperture into said chamber, an aperture in said bottom wall, a gridpositioned at the top of said ion chamber for repelling ions through theaperture in the bottom wall of said chamber, a deflection electrodepositioned adjacent said grid on the far side of said grid from thebottom wall of said chamber, said deflection electrode being physicallyspaced from said grid and in parallel relation thereto, and operativetherethrough for controlling the trajectories of the electron stream insaid chamber.

8. The ion source as is set forth in claim 7, further includingadjustable means for supplying voltages to said repeller electrode andto said deflection electrode to control the trajectories of the electronstream in said cham ber for optimizing the production of positive ionsin said chamber.

9. The ion source as is set forth in claim 7, further including a heaterpositioned behind said deflection electrode for indirectly heating saidsecond focus electrode and said ion source, and means including a switchfor energizing said heater.

10. The ion source as is set forth in claim 7, wherein said gridcomprises a metal frame having a plurality of spaced wire portionsstretched across said frame in at least one plane and firmly anchored tosaid frame.

11. The ion source as set forth in claim 7, said means for producingelectrons comprising a filament having a width substantially equal tothe height of the electron beam aperture in the side wall of the ionchamber.

References Cited by the Examiner UNITED STATES PATENTS 2,694,152 11/1954Berry 3l3230 X 2,732,500 1/1956 McLaren 25041.9 2,967,239 1/1961 Zarnany25041.9

JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, S. SCHLOSSER,

Assistant Examiners.

1. AN ION SOURCE COMPRISING AN ION CHAMBER, MEANS INCLUDING A FILAMENTAND FIRST FOCUSING ELECTRODE FOR PRODUCING AND DIRECTING A STREAM OFELECTRONS INTO SAID CHAMBER, AN ION OUTLET ON ONE SIDE OF SAID CHAMBER,A GRID POSITIONED ON THE OPPOSITE SIDE OF SAID CHAMBER FOR REPELLINGIONS TOWARD SAID ONE SIDE, AND A DEFLECTION ELECTRODE POSITIONED ON THEFAR SIDE OF SAID GRID FROM SAID ONE SIDE OF SAID CHAMBER ANDSUBSTANTIALLY PARALLEL TO SAID GRID FOR CONTROLLING THE TRAJECTORIES OFELECTRONS IN SAID CHAMBER TO INCREASE THE AMOUNT OF POSITIVE IONS WHICHARE PRODUCED IN SAID CHAMBER, SAID DEFLECTION ELECTRODE IMPOSINGOPERATIVE FIELD FORCES IN SAID CHAMBER THROUGH THE INTERPOSED GRID.